(a) Field of the Invention
The present invention relates to linearizers that are used in communication systems and, more particularly, to a method of designing linearizers.
(b) Description of Related Art
Communication systems utilize a wide range of transmission equipment for establishing communication links. Specifically, most communication systems utilize power amplifiers to boost a signal to an acceptable power level for transmission. Communication systems use a broad array of amplifiers including tube amplifiers and transistor amplifiers. Each particular amplifier has unique gain and phase distortion characteristics. The gain of an amplifier is defined as the ratio of the output power to the input power. The gain of an amplifier may be plotted as output power against input power. The gain of an amplifier is typically linear at low input powers, but non-linear as the input signal power is increased. As the input power increases, the amplifier is operated close to saturation and the gain of the amplifier begins to fall off. A criteria for defining the gain performance of an amplifier is through the use of the amplitude modulation to amplitude modulation (AM/AM) conversion coefficient. The AM/AM conversion coefficient measures the slope of an amplifier's gain plot. As the input signal to the amplifier gets larger (i.e., the amplifier is operated closer to saturation) the AM/AM conversion coefficient becomes smaller.
Another criteria for defining amplifier performance is by measuring the phase differential between the input and the output of an amplifier. The phase differential of an amplifier is typically plotted against input power supplied to the amplifier. At low input power levels, the phase differential of an amplifier may be constant. However, as the amplifier input becomes larger and as the amplifier operates closer to its saturation point the phase differential typically becomes non-linear. A standard measure of phase differential against input power is the amplitude modulation to phase modulation (AM/PM) conversion coefficient. The AM/PM conversion coefficient is the slope of the relative phase against the input power supplied to the amplifier. The AM/PM conversion coefficient is typically constant at low amplifier input power and increases with an increase in input power.
It is desirable to eliminate or minimize the non-linear characteristics of an amplifier. Elimination of the non-linear characteristics allows an amplifier to operate close to its saturation point, which yields cost, power consumption, and size reductions in amplifier circuitry. The use of a linearizer to minimize the non-linear effects of an amplifier is known. A linearizer is typically located in an amplifier lineup before, or combined with, a preamplifier. The linearizer produces amplitude and phase distortion of the input signal in order to compensate for the non-linear characteristics of the power amplifier. That is, the linearizer and the amplifier, when cascaded, produce a linear output characteristic.
Traditionally, the design of a linearizer/amplifier system has been done manually. That is, linearizer parameters (e.g., resistance, capacitance, and gain) were adjusted and the linearizer/amplifier characteristics (e.g., AM/AM and AM/PM) were monitored either using software simulations or laboratory measurements. After several iterations of the tuning process on a parameter, it is usually up to the discretion of the designer whether to keep adjusting the same parameter or switch to the next parameter. This iterative and indefinite process is very time consuming and satisfactory results are not guaranteed. If a microstrip linearizer is being analyzed, the tuning and adjustment of linearizer parameters is very difficult and inaccurate. It would be desirable to have an automatic and iterative simulation method for adjusting linearizer parameters to achieve desired AM/AM and AM/PM characteristics without the need to engage in trial and error methods in a laboratory.